Prevalence studies for HIV-1 infection in newborns are difficult to obtain since the commonly used antibody assays are not appropriate for testing babies. Babies carry the maternal antibody for up to 12-18 months after birth, so that all babies born to seropositive mothers will also test positive. Yet 40-50% of these babies are not actually infected. In this grant, we propose to use Polymerase Chain Reaction (PCR) assays to examine the lymphocytes of babies born to HIV-1 seropositive mothers for the presence of the HIV-1 genome. This is an extremely sensitive effect test for the virus, and is capable of detecting even 10 viral genomes in less than 1 ml of blood. Peripheral blood lymphocytes will be collected from about 80 babies born to HIV-1 seropositive mothers. Samples collected at birth and at 3-month follow-up intervals will be tested by PCR and Western Blot assays. The results of the PCR will be correlated with the subsequent loss of maternal antibody and seroconversion to baby antibody. These results will determine whether PCR data can be used as a reliable indicator of actual infection and a predictor of subsequent seroconversion. These studies will provide epidemiological data for the incidence of HIV-1 infection in babies born to seropositive mothers, and provide a basis for predictive tests for AIDS in these babies.